I was at MWS today to collect my re-sealed and repaired tyre. The technician said he’d inflated the tyre to 30psi. I told him Morgan say 22psi all round and his comment was that he wouldn’t trust their calculations: to him that is too low, especially if the car is loaded.
So I did some web research and from the Caravan Club of all places got the information that resulted in these calculation
First you need you tyre. Note the data on the side: maximum load and maximum pressure.
For the 205 x 55 R16 on the roadster this is 615kg and 51psi.
Now calculate the typical loaded weight of your Mog.
For my Roadster solo I estimate this at 1100kg, based on Morgan’s stated figure of 1000kg with tools and fuel and 100kg for me and my junk.
Now divide the loaded weight by 4 to get the load per tyre. Morgan give, for a max permissible weight, a front/rear bias of 40%/60%.
Given all additional load added is mostly over the rear I’m estimating the Front/rear of a car with 1 person and no proper luggage as 45%/55% so this means:
Normal Load: (45% / 55%)
Front axle this is 495kg or 247.5kg per wheel
Rear axle this is 605kg or 302.5kg per wheel
Actually if you want to be really pedantic for solo use you can bias the weight distribution left/right as well as front/back but do you want to have 4 pressures to remember?
Maximum Load: (40% / 60%)
Front axle this is 560kg or 280kg per wheel
Rear axle this is 840kg or 420kg per wheel
Notice that the maximum permitted load is well below the 615Kg the tyre allows. Actually this means a lower load capacity tyre could be used, normally it is set at 110% of the maximum permitted so this is 462kg. This is an “81” code tyre, but no one makes such a tyre in the size needed.
Finally work out the ideal tyre pressure take the figure for the maximum pressure for the tyre and divide it by the maximum weight for the tyre.
Multiply this figure by the actual load you are going to put on the tyre and you have the correct pressure.
So:
Normal Load: (45% / 55%)
51 psi / 615kg = 0.082927
Front axle is 247.5kg per wheel so 20.5psi
Rear axle 302.5kg per wheel so 25 psi
Maximum Load: (40% / 60%)
Front axle is 280kg per wheel so 23.2 psi
Rear axle is 420kg per wheel so 34.8 psi.
Morgan state 22psi all round. For a normal load it is probably OK, but for a heavily loaded car it is worryingly low at the back.
I’ve set Aldermog at 22 front and 27.5 rear as 2 up with stuff we run at about 1250kg. It wil be interesting to see if there is any difference…..!!
Last edited by Aldermog; 15/08/14 05:33 PM.
